Mirrors used in high-performance Fabry-Perot (FP) spectrometers for visible and infrared wavelengths often contain multi-layer thin film dielectric stacks to achieve high reflectivity. These thin film mirrors are fabricated using a number of standard vacuum deposition techniques such as evaporation, sputtering or chemical vapor deposition. For teraHertz (THz) optics, however, thin film deposition is challenging since layer thickness scales with increasing wavelength. As the wavelength increases so does the thickness of the deposition layers. Individual layers are typically thicker than 10 microns, depending on wavelength range and refractive index. When attempting to form thicker dielectric films, greater than 10 micron thickness, thin film deposition methods are very time consuming or impractical and, disadvantageously, result in high in-plane stresses. These high stresses result in film cracks and bowed, or curved mirrors.
THz mirrors may be individually assembled from very thin single-crystal silicon layers that are separated from each other by air gaps. These air gaps are defined by mechanical spacers that are made from Mylar, brass, or silicon. In general, the thin silicon layers are stacked one on top of the other, with an air gap between each silicon layer formed by the mechanical spacers. In theory, such THz mirrors may have good wavelength selectivity and optical performance when used for Fabry-Perot spectrometers, lasers, or other THz optical systems. Due to the very large refractive index difference between silicon (n=3.38) and air (n=1), high reflectivity mirrors may be provided using only a few silicon-air gap layer pairs.
In practice, however, conventional assembly processes yield poor optical performance when applied to Fabry-Perot THz mirrors. Using conventional assembly approaches, it is difficult to precisely control the spacing and flatness of a stack of thin and flexible silicon layers, on the order of 10 microns thickness. Single crystal silicon in such thin thickness range is also fragile and brittle, making it difficult to handle.
As disclosed in U.S. Pat. Nos. 7,864,326 and 8,198,590, semiconductor and micro-electromechanical system (MEMS) wafer processing techniques can be used to fabricate THz mirrors with alternating layers of silicon and air. The air gaps are provided by precise spacing of thin silicon supports.
The wafer processing approach leverages unique tools and techniques for repeatable manufacturing of these THz mirrors, thus providing better structural control and lower cost than a conventional assembly process. Furthermore, unlike conventional assembly processes in which devices or components are fabricated serially, one at a time, wafer-level fabrication enables manufacturing of many devices or components simultaneously, in parallel, on a single silicon wafer. The wafer-level fabrication processes for THz mirrors include photolithography and DRIE (deep reactive ion etching) for defining the thin silicon layers and the spacer supports for the air gaps. The fabrication processes include dicing to create individual mirror periods, as well as stacking and bonding the mirror periods together to form the THz mirrors.
The MEMS wafer processing approach, described in the two aforementioned US Patents, provides some improvement in stack spacing and flatness control; however, there are still significant fabrication challenges because of the required wafer thickness. For example, for a mirror design centered at 1 THz, the wafer thickness is approximately 100 microns for a preferred quarter-wave mirror stack design, including both the thin silicon mirror and the spacer support for providing separation control. In general, such thickness of flexible silicon is not practical to handle in manufacturing, which normally uses 150 mm to 300 mm diameter silicon wafers.
Large FP mirrors, such as those needed for use in THz tunable filters with imaging focal plane arrays (FPAs), require precise control of layer to layer separation and good flatness over large optical apertures. These optical apertures may be several centimeters in diameter. The FP mirror components also need to be robust enough so as not to break during fabrication and final assembly. Both the conventional assembly process and wafer-level fabrication suffer from significant challenges in achieving performance and manufacturability requirements for large THz FP mirrors.
There is a need, therefore, for a THz Fabry-Perot tunable filter that has a large optical aperture, good optical performance, and is robust through fabrication and final assembly.